Abstract: In the paper,the weld bond and its microstructures of a dissimilar metal welded joint of C-Mn-Ti-B ultra strength m steel and γ-steel were investigated.It was found that microhardness (HV_0.05) in the weld bond and austenite-rich zone was larger than the one in the quenched zone near the fusion line.And the results shown that chemical compositions in austenite-rich zone were almost the same as those in composite weld metal and the formation of austenite-rich zone was dependent both on its chemical components and on its cooling rate.Considering weld thermal cycle,diffusion of alloy elements such as C、B、Cr、Ni、Fe,chemical elements distributions and microstructures of the joint,a model which can be used to explain the formation of the weld bond and its microstructures transformation was presented.According to the model,a welded joint during welding possessed three zones:liquid zone,liquid+solid coexistent zone and solid zone.During consequent cooling,the metal in liquid zone became the composite weld metal (α+γ) and austenite-rich zone,the metal in liquid+solid zone formed the weld bond,the metal in solid zone transformed into true HAZ and parent metal unaffected by welding heat.Based on the model,martensite like structure and formation of (Cr、Fe)₂₃(C、B)₆ Boride in the weld bond can be explained successfully.